Diesel engines operate by compression ignition. They have compression ratios in the range of 14:1 to 17:1 or higher and for that reason obtain more useful work from a given amount of fuel compared to a spark-ignited engine. Historically, diesel engines have been operated on a petroleum-derived liquid hydrocarbon fuel boiling in the range of about 300.degree.-750.degree. F. Recently, because of dwindling petroleum reserves, alcohol and alcohol-hydrocarbon blends have been studied for use as diesel fuel.
One major factor in diesel fuel quality is cetane number. Cetane number is related to ignition delay after the fuel is injected into the combustion chamber. If ignition delay is too long, the amount of fuel in the chamber increases and upon ignition results in a rough running engine and increased smoke. A short ignition delay results in smooth engine operation and decreases smoke. Commercial petroleum diesel fuels generally have a cetane number of about 40-55. Alcohols have a much lower cetane value and require the addition of a cetane improver for successful engine operation.
Through the years, many types of additives have been prepared to raise the cetane number of diesel fuel. These include peroxides, nitrites, nitrates, nitrocarbamates, and the like. Alkyl nitrates such as amyl nitrate, hexyl nitrate and mixed octyl nitrates have been used commercially with good results. Likewise certain cyclohexyl nitrates and alkoxyalkyl nitrates have been suggested as cetane improvers for diesel fuel (Olin et al U.S. Pat. No. 2,294,849).
Unfortunately some compounds that are very effective cetane improvers are also fairly sensitive explosives. Because of this they have not found commercial acceptance. Attempts have been made to desensitize some of these explosive compounds by blending with inert solvents. However, such blends are much less effective than the original compound and would require shipping and storing large amounts of cetane improver additive to provide the required cetane boost.